Dopamine modulates the optomotor response to unreliable visual stimuli in Drosophila melanogaster

Combinatory Actions of Co-transmitters in Dopaminergic Systems Modulate Drosophila Olfactory Memories

Dopamine neurons (DANs) are extensively studied in the context of associative learning, in both vertebrates and invertebrates. In the acquisition of male and female Drosophila olfactory memory, the PAM cluster of DANs provides the reward signal, and the PPL1 cluster of DANs sends the punishment signal to the Kenyon cells (KCs) of mushroom bodies, the center for memory formation. However, thermo-genetical activation of the PPL1 DANs after memory acquisition impaired aversive memory, and that of the PAM DANs impaired appetitive memory. We demonstrate that the knockdown of glutamate decarboxylase, which catalyzes glutamate conversion to GABA in PAM DANs, potentiated the appetitive memory. In addition, the knockdown of glutamate transporter in PPL1 DANs potentiated aversive memory, suggesting that GABA and glutamate co-transmitters act in an inhibitory manner in olfactory memory formation. We also found that, in γKCs, the Rdl receptor for GABA and the mGluR DmGluRA mediate the inhibition. Although multiple-spaced training is required to form long-term aversive memory, a single cycle of training was sufficient to develop long-term memory when the glutamate transporter was knocked down, in even a single subset of PPL1 DANs. Our results suggest that the mGluR signaling pathway may set a threshold for memory acquisition to allow the organisms' behaviors to adapt to changing physiological conditions and environments.SIGNIFICANCE STATEMENT In the acquisition of olfactory memory in Drosophila, the PAM cluster of dopamine neurons (DANs) mediates the reward signal, while the PPL1 cluster of DANs conveys the punishment signal to the Kenyon cells of the mushroom bodies, which serve as the center for memory formation. We found that GABA co-transmitters in the PAM DANs and glutamate co-transmitters in the PPL1 DANs inhibit olfactory memory formation. Our findings demonstrate that long-term memory acquisition, which typically necessitates multiple-spaced training sessions to establish aversive memory, can be triggered with a single training cycle in cases where the glutamate co-transmission is inhibited, even within a single subset of PPL1 DANs, suggesting that the glutamate co-transmission may modulate the threshold for memory acquisition.

Visual processing in the fly, from photoreceptors to behavior

Originally a genetic model organism, the experimental use of Drosophila melanogaster has grown to include quantitative behavioral analyses, sophisticated perturbations of neuronal function, and detailed sensory physiology. A highlight of these developments can be seen in the context of vision, where pioneering studies have uncovered fundamental and generalizable principles of sensory processing. Here we begin with an overview of vision-guided behaviors and common methods for probing visual circuits. We then outline the anatomy and physiology of brain regions involved in visual processing, beginning at the sensory periphery and ending with descending motor control. Areas of focus include contrast and motion detection in the optic lobe, circuits for visual feature selectivity, computations in support of spatial navigation, and contextual associative learning. Finally, we look to the future of fly visual neuroscience and discuss promising topics for further study.

Low-nutrient diet in Drosophila larvae stage causes enhancement in dopamine modulation in adult brain due epigenetic imprinting

Nutrient scarcity is a frequent adverse condition that organisms face during their development. This condition may lead to long-lasting effects on the metabolism and behaviour of adults due to developmental epigenetic modifications. Here, we show that reducing nutrient availability during larval development affects adult spontaneous activity and sleep behaviour, together with changes in gene expression and epigenetic marks in the mushroom bodies (MBs). We found that open chromatin regions map to 100 of 241 transcriptionally upregulated genes in the adult MBs, these new opening zones are preferentially located in regulatory zones such as promoter-TSS and introns. Importantly, opened chromatin at the Dopamine 1-like receptor 2 regulatory zones correlate with increased expression. In consequence, adult administration of a dopamine antagonist reverses increased spontaneous activity and diminished sleep time observed in response to early-life nutrient restriction. In comparison, reducing dop1R2 expression in MBs also ameliorates these effects, albeit to a lesser degree. These results lead to the conclusion that increased dopamine signalling in the MBs of flies reared in a poor nutritional environment underlies the behavioural changes observed due to this condition during development.

Interneurons of fan-shaped body promote arousal in Drosophila

Sleep is required to maintain physiological functions and is widely conserved across species. To understand the sleep-regulatory mechanisms, sleep-regulating genes and neuronal circuits are studied in various animal species. In the sleep-regulatory neuronal circuits in Drosophila melanogaster, the dorsal fan-shaped body (dFB) is a major sleep-promoting region. However, other sleep-regulating neuronal circuits were not well identified. We recently found that arousal-promoting T1 dopamine neurons, interneurons of protocerebral bridge (PB) neurons, and PB neurons innervating the ventral part of the FB form a sleep-regulatory circuit, which we named "the PB-FB pathway". In the exploration of other sleep-regulatory circuits, we found that activation of FB interneurons, also known as pontine neurons, promoted arousal. We then found that FB interneurons had possible connections with the PB-FB pathway and dFB neurons. Ca2+ imaging revealed that FB interneurons received excitatory signals from the PB-FB pathway. We also demonstrated the possible role of FB interneurons to regulate dFB neurons. These results suggested the role of FB interneurons in sleep regulation.

A non-canonical on-demand dopaminergic transmission underlying olfactory aversive learning

Dopamine (DA) is involved in various brain functions including associative learning. However, it is unclear how a small number of DA neurons appropriately regulates various brain functions. DA neurons have a large number of release sites and release DA non-specifically to a large number of target neurons in the projection area in response to the activity of DA neurons. In contrast to this "broad transmission", recent studies in Drosophila ex vivo functional imaging studies have identified "on-demand transmission" that occurs independent on activity of DA neurons and releases DA specifically onto the target neurons that have produced carbon monoxide (CO) as a retrograde signal for DA release. Whereas broad transmission modulates the global function of the target area, on-demand transmission is suitable for modulating the function of specific circuits, neurons, or synapses. In Drosophila olfactory aversive conditioning, odor and shock information are associated in the brain region called mushroom body (MB) to form olfactory aversive memory. It has been suggested that DA neurons projecting to the MB mediate the transmission of shock information and reinforcement simultaneously. However, the circuit model based on on-demand transmission proposes that transmission of shock information and reinforcement are mediated by distinct neural mechanisms; while shock transmission is glutamatergic, DA neurons mediates reinforcement. On-demand transmission provides mechanical insights into how DA neurons regulate various brain functions.

A connectome of the Drosophila central complex reveals network motifs suitable for flexible navigation and context-dependent action selection

Flexible behaviors over long timescales are thought to engage recurrent neural networks in deep brain regions, which are experimentally challenging to study. In insects, recurrent circuit dynamics in a brain region called the central complex (CX) enable directed locomotion, sleep, and context- and experience-dependent spatial navigation. We describe the first complete electron-microscopy-based connectome of the Drosophila CX, including all its neurons and circuits at synaptic resolution. We identified new CX neuron types, novel sensory and motor pathways, and network motifs that likely enable the CX to extract the fly's head-direction, maintain it with attractor dynamics, and combine it with other sensorimotor information to perform vector-based navigational computations. We also identified numerous pathways that may facilitate the selection of CX-driven behavioral patterns by context and internal state. The CX connectome provides a comprehensive blueprint necessary for a detailed understanding of network dynamics underlying sleep, flexible navigation, and state-dependent action selection.

Multi-regional circuits underlying visually guided decision-making in Drosophila

Visually guided decision-making requires integration of information from distributed brain areas, necessitating a brain-wide approach to examine its neural mechanisms. New tools in Drosophila melanogaster enable circuits spanning the brain to be charted with single cell-type resolution. Here, we highlight recent advances uncovering the computations and circuits that transform and integrate visual information across the brain to make behavioral choices. Visual information flows from the optic lobes to three primary central brain regions: a sensorimotor mapping area and two 'higher' centers for memory or spatial orientation. Rapid decision-making during predator evasion emerges from the spike timing dynamics in parallel sensorimotor cascades. Goal-directed decisions may occur through memory, navigation and valence processing in the central complex and mushroom bodies.